Autonomic Nervous System - Lecture Notes Flashcards

Somatic Nervous System

Autonomic nervous system includes visceral motor neurons, providing involuntary control of visceral organs (cardio, respiratory, digestive, urinary, reproductive).
Autonomic ganglia are cell body clusters of visceral motor neurons located outside the CNS.
Preganglionic neurons have cell bodies in brainstem and spinal cord and project to autonomic ganglia.
Postganglionic neurons have cell bodies in autonomic ganglia and synapse with peripheral target organs.

Function: “Fight or flight” — prepares the body to deal with stress.
Effects include heightened mental alertness, increased metabolic rate, reduced digestive and urinary functions, activation of energy reserves, increased respiratory rate, increased heart rate and blood pressure, activation of sweat glands, and pupil dilation.
Preganglionic neurons: short; cell bodies in thoracic and lumbar regions; release acetylcholine (ACh).
Sympathetic ganglia: clusters of postganglionic neuron cell bodies.
- Sympathetic chain ganglia: on either side of the vertebral column.
- Collateral ganglia: anterior to vertebral column (celiac, superior mesenteric, inferior mesenteric).
- Adrenal medulla: neurotransmitters function as hormones; effects last much longer than direct sympathetic innervation.
Postganglionic neurons: long; cell bodies in ganglia near the spinal cord; release norepinephrine (NE).

Cholinergic synapses: use acetylcholine (ACh); effects are always excitatory.
Adrenergic synapses: use norepinephrine (NE); effects can be excitatory or inhibitory.

Adrenergic receptors: membrane G-protein coupled receptors; bind to norepinephrine and epinephrine.
Alpha receptors:
- Alpha-1 Receptors: more common; in smooth muscle cells; stimulation has excitatory effect via release of intracellular Ca^{2+} ions.
- Alpha-2 Receptors: found on pre- and post-synaptic sympathetic and parasympathetic neurons; coordinates activity of the ANS; stimulation lowers [cAMP] levels in cytoplasm → inhibitory effect.
Beta receptors:
- Beta-1: increased metabolic activity; effects on heart and muscles; excitatory.
- Beta-2: dilation of respiratory pathways; inhibitory.
- Beta-3: lipolysis (breakdown of triglycerides in adipocytes).

Function: “Rest and digest” — conserves energy and maintains resting metabolic rate.
Effects include decreased metabolic rate, decreased heart rate and blood pressure, increased secretion by salivary and digestive glands, increased motility and blood flow in digestive tract, stimulation of urination and defecation, and pupil constriction.
Preganglionic neurons: long; cell bodies in brainstem and sacral regions; release acetylcholine (ACh).
Postganglionic neurons: short; cell bodies in ganglia with target organs; release acetylcholine (ACh).
Vagus nerve: 75% of all parasympathetic output.

All parasympathetic neurons release ACh → cholinergic synapses.
Acetylcholine is inactivated at the synapse by acetylcholinesterase; ACh diffused to surrounding tissues is inactivated by tissue cholinesterase.

Nicotinic receptors: chemically gated Na^{+} channels; activation by ACh causes excitation of the postganglionic neuron; binds to nicotine.
Muscarinic receptors: G-protein-coupled receptors; response can be excitatory or inhibitory; binds to muscarine (found in some poisonous mushrooms).

Sympathetic (thoracolumbar) vs Parasympathetic (craniosacral): Fight or flight vs Rest and digest.
Preganglionic fibers: short (sympathetic) vs long (parasympathetic); release ACh in both cases.
Cell body locations: sympathetic in thoracic and lumbar regions; parasympathetic in brainstem and sacral regions.
Postganglionic fibers: long (sympathetic) vs short (parasympathetic); sympathetic postganglionic neurons often release NE; parasympathetic postganglionic neurons release ACh.
Target organ effects vary: sympathetic often dilates pupils, inhibits salivation, accelerates heartbeat, dilates bronchi, inhibits digestion, promotes glucose release, and inhibits bladder constriction; parasympathetic constricts pupils, stimulates salivation, slows heartbeat, constricts bronchi, stimulates digestion, stimulates bile release, and contracts the bladder.

Preganglionic neurons: long; cell bodies in brainstem and sacral regions; release ACh.
Postganglionic neurons: short; cell bodies in ganglia with target organs; release ACh.
Vagus nerve contributes a major portion of parasympathetic output (≈ 75\%).
All parasympathetic neurons are cholinergic (ACh-releasing).
ACh is rapidly inactivated at the synapse by acetylcholinesterase; tissue cholinesterase also contributes to inactivation outside the synapse.
Nicotinic receptors are ligand-gated Na^{+} channels; Muscarinic receptors are GPCRs with variable effects.
Distinct pathways in autonomic control reflect the need for rapid, localized or widespread regulatory effects depending on the organ, with the adrenal medulla providing a hormonal analog of sympathetic signaling.